If I understand your use of appearance: none correctly, it seems like a neat pattern. It took me a few minutes to work out, though, which makes me think it might be susceptible to removal by future maintainers. Is it a common pattern in CSS testing?

If not, a comment would go a long way. Something like, "For most elements, the expected behavior identical to the initial value. Initially set the property value to none to verify that this is value is being interpreted (instead of simply ignored)."

I don't know if it's common in the css testsuite to rely on this particular behavior, but regardless, a comment seems like a good idea.

I think it could be stated more simply

/* If the value being tested is not supported, then 'none' is used instead, which is intended to fail the test. */

Also I think this test needs

• rebasing
• change -webkit-appearance to appearance
• run the build script to generate the -webkit-appearance variant of this test

I followed up with @zcorpan's suggestions. That makes me a co-author, so I'd like further review before merging. @frivoal @mrego @plinss @tantek would one of you mind giving this a look?

(The prior version of this branch is available here.)

The change LGTM, @jugglinmike . I can't mark as reviewed, but I can admin-merge.

Thanks!